The higher the D.O.T. level the more frequently you have to change the fluid because it sucks up more moisture faster than the lower D.O.T. level.
DOT 3 last longer but has a lower boiling point. DOT 5 absorbs more water faster but has a higher boiling point. Most street cars won't hit the boiling point unless you are Autocrossing or pushing it big time to get it up into those temperatures.
-Andy
